Cathode luminescence characteristics of ZnGa2O4 phosphor thin films with the doped activator

The zincgallate (ZnGa₂O₄) phosphor thin film was grown using RF magnetron sputtering system at various process parameters. A ZnGa₂O₄ phosphor thin film was deposited on Si(1 0 0) substrate and annealed by a rapid thermal processor (RTP). The X-ray diffractometer (XRD) patterns indicate that the Mn-doped ZnGa₂O₄ phosphor thin film shows a (3 1 1) main peak and a spinel phase. A ZnGa₂O₄ phosphor thin film has better crystallization due to increased substrate, annealing temperature and deposition time. Also the ZnGa₂O₄:Mn phosphor thin film shows green emission (510 nm, ⁴T₁→⁶A₁), and the ZnGa₂O₄:Cr phosphor thin film shows red emission (705 nm, ⁴A₂→⁴T₂).     

Effect of nanoscaled SnO2 coating on ZnS:Mn phosphors under electron irradiation

The luminescence properties of SnO₂-coated ZnS:Mn phosphors are investigated. In the case of photoluminescence, emission intensities show little change when SnO₂ is coated on the surface of ZnS:Mn, while in the case of cathodoluminescence (CL), emission intensities vary depending on excitation energies. In order to determine the luminescence behaviors, surface analyses of the phosphors were performed. Auger electron spectroscopy showed that the width of the SnO₂ layer on the ZnS:Mn phosphor was saturated at approximately 120 nm. Also, X-ray photoelectron spectroscopy indicated that the SnO₂ layers are well formed and saturated when the molar ratios of Sn/Zn are larger than 0.005. These results suggest that the changes in the CL emissions can be attributed to a lowering of the junction barrier.     

Studies of minority carrier transport in ZnO

The temperature dependence of the minority carrier diffusion length and lifetime in bulk n-type ZnO was studied using Electron-Beam-Induced Current (EBIC) and cathodoluminescence (CL) techniques. The diffusion length was observed to increase exponentially over the temperature range from 25 to 125 C, yielding an activation energy of 45 ± 2 meV. A concomitant decrease of the cathodoluminescence intensity for the near-band-edge transition was also observed. The activation energy determined by optical measurements was 58 ± 7 meV. The larger minority carrier diffusion length and smaller luminescence intensity are attributed to the increased lifetime of non-equilibrium holes in the valence band at elevated temperatures. Carrier trapping on Li-related levels with activation energy 283 ± 9 meV is also addressed.     

Effect of size variation on the cathodoluminescence characteristics of graphene quantum dots

Cathodoluminescence (CL) has been studied in graphene quantum dots (GQDs) by varying their average size (d) from 5 to 35 nm. The size dependence of CL peak wavelength is very analogous to that of photoluminescence (PL) peak wavelength unusually showing non-monotonic behaviors having a maximum at d = ∼17 nm. The CL behaviors can therefore be attributed to the novel feature of GQDs, i.e., the circular-to-polygonal-shape and corresponding edge-state variations of GQDs at d = ∼17 nm as d increases. However, the peak wavelengths of CL are especially much smaller than those of PL at both ends in the size range of GQDs, possibly resulting from the recombination of the electron-beam-excited e-h pairs at higher energy states before thermalization due to fast carrier-carrier scattering dominating over electron-phonon scattering in graphene.     

UV cathodoluminescence of crystalline α-quartz at low temperatures

Two luminescence bands in the UV range were detected in crystalline α-quartz under electron beam excitation (6 kV, 3-5 μA). One band is situated at 5 eV and could be observed in pure samples. Its intensity increases with cooling below 100 K and undergoes saturation below 40 K alongside a slow growth with the time of irradiation at 9 K. The decay curve of the band at 5 eV contains two components, a fast (<10 ns) and a slow one in the range of 200 μs. The photoluminescence band at 5 eV with a similar temperature dependence was found in previously neutron-irradiated crystalline α-quartz. Therefore, the band at 5 eV was attributed to host material defects in both irradiation cases. The creation mechanism of such defects by electrons, the energy of which is lower than the threshold for a knock-out mechanism of defect creation, is discussed. Another band at 6 eV, containing subbands in different samples, appears in the samples containing aluminum, lithium and sodium ions. This luminescence is ascribed to a tunnel radiative transition in an association of (alkali atom)⁰-[AlO₄]⁺ that is formed after the trapping of an electron and a hole by Li⁺ (or Na⁺) and AlO₄.     

Spatial resolution of imaging contaminations on the GaAs surface by scanning tunneling microscope-cathodoluminescence spectroscopy

We obtained the luminescence image of the GaAs (1 1 0) surface by scanning tunneling microscope-cathodoluminescence (STM-CL) spectroscopy, where low-energy (∼100 eV) electrons field emitted from the STM tip were used as a bright excitation source. The STM-CL image with high photon signal (1.25 × 10⁴ cps) showed the dark image corresponding to the surface contamination in the STM image working as the nonradiative recombination centers of carriers. This dark image demonstrated the spatial resolution of about 100 nm in STM-CL spectroscopy of the GaAs (1 1 0) surface, which was determined by the field-emitted electron beam diameter.     

Defect detection in semiconductor layers with built-in electric field with the use of cathodoluminescence

Cathodoluminescence (CL) in scanning electron microscopy (SEM) is commonly accepted as revealing local properties of a specimen region illuminated by an electron beam. CL is widely used to visualize defects in semiconductor structures. However, the presence of a strong electric field in, for example, heterojunctions or p–n junctions causes a separation of generated electron–hole (e–h) pairs and suppresses recombination in the specimen region excited by the beam. As a result CL – a radiative recombination – becomes quenched. At the same time, electron beam-induced current (EBIC) flows throughout the structure, which may produce secondary electroluminescence that is registered by the CL detector. Consequently, the CL measurement is distorted and if there are defects in the structure, they remain unrevealed. The current study shows that registration of the CL signal for different values of electron beam current (including high ones) enables true defect detection in semiconductor layers with built-in electric field. Results for a special test structure prepared with focused ion beam on AlGaAs/GaAs laser heterostructures with an 8 nm InGaAs quantum well are presented.     

Reduction of Eu to Eu in MAl2Si2O8 (M=Ca, Sr, Ba) in air condition

In general, the reduction of Eu³⁺ to Eu²⁺ in solids needs an annealing process in a reducing atmosphere. In this paper, it is of great interest and importance to find that the reduction of Eu³⁺ to Eu²⁺ can be realized in a series of alkaline-earth metal aluminum silicates MAl₂Si₂O₈ (M=Ca, Sr, Ba) just in air condition. The Eu²⁺-doped MAl₂Si₂O₈ (M=Ca, Sr, Ba) powder samples were prepared in air atmosphere by Pechini-type sol-gel process. It was found that the strong band emissions of 4f⁶5d¹-4f⁷ from Eu²⁺ were observed at 417, 404 and 373 nm in air-annealed CaAl₂Si₂O₈, SrAl₂Si₂O₈ and BaAl₂Si₂O₈, respectively, under ultraviolet excitation although the Eu³⁺ precursors were employed. In addition, under low-voltage electron beam excitation, Eu²⁺-doped MAl₂Si₂O₈ also shows strong blue or ultraviolet emission corresponding to 4f⁶5d¹-4f⁷ transition. The reduction mechanism from Eu³⁺ to Eu²⁺ in these compounds has been discussed in detail.     

Volcanic Quartz Growth Zoning Identified by Cathodoluminescence and EPMA Studies

 Quartz is a common phenocryst in acidic volcanic rocks but rarely contributes to the understanding of the magmatic or eruptive processes due to its apparent chemical homogeneity. Cathodoluminescence studies indicate that volcanic quartz is strongly zoned with respect to the trace elements. The determination of this zoning can help to elucidate the crystallisation history of the magmatic rock. Polished thin sections from rhyolitic and dacitic volcanic rocks were examined by cathodoluminescence mounted on an optical microscope and analysed by electron probe microanalysis. X-ray mapping of the trace elements incorporated in the zoned quartz was also performed. The interpretation of the data obtained from these different analytical methods provides useful information on the chemical zoning of single crystals and therefore on the chemico-physical conditions of the melt from which the quartz crystallised. Quartz from dacitic rock shows a complex cathodoluminescence oscillatory zoning, consisting of large red-brown and blue bands concentric to a resorbed core. Quartz from rhyolite shows simpler cathodoluminescence zoning, consisting of dark to bright blue luminescent bands. The EMPA analyses and the X-ray maps combined with the cathodoluminescence images suggest a clear correlation between the concentrations of some trace element (mainly Al and Ti, acting as activators of luminescence) and the cathodoluminescence colours pattern zoning. The incorporation of these trace elements during the growth of the crystals is linked to small variations in the thermo-chemical properties of the magma such as temperature and chemical composition.     

Synthesis, crystal structure and characterization of iron pyroborate (Fe2B2O5) single crystals

Single crystals of iron(II) pyroborate, Fe₂B₂O₅, were prepared at 1000–1050 °C under an argon atmosphere. The crystals were transparent, yellowish in color and needle-like or columnar. The crystal structure of Fe₂B₂O₅ was analyzed by single-crystal X-ray diffraction. Refined triclinic unit cell parameters were a=3.2388(2), b=6.1684(5), c=9.3866(8) Å, α=104.613(3)°, β=90.799(2)° and γ=91.731(2)°. The final reliability factors of refinement were R1=0.020 and wR2=0.059 [I > 2σ(I)]. Transmittance over 50% in the visible light region from 500 to 750 nm was observed for a single crystal of Fe₂B₂O₅ with a thickness of about 0.3 mm. The light absorption edge estimated from a diffuse reflectance spectrum was at around 350 nm (3.6 eV). Magnetic susceptibility was measured for single crystals at 4–300 K. Fe₂B₂O₅ showed antiferromagnetic behavior below the Néel temperature, T_N≈70 K, and the Weiss temperature was T_W=36 K. The effective magnetic moment of Fe was 5.3μ_B.